In many applications, the use of electric motors represents a simple and effective method for delivering mechanical power for traction and/or propulsion systems. This is especially the case for devices such as small-scale single-rotor and multi-rotor unmanned aerial vehicles (“UAVs”).
A variety of energy storage methods have been used to provide the power necessary for electric motor driven traction and propulsion applications and examples include, but are not limited to: petrol-electric or diesel-electric powertrains; turbine-electric powertrains; and batteries.
In petrol-electric and diesel-electric powertrains, a prime mover combusts petroleum based fuel (typically liquid) to create mechanical energy and that mechanical energy is then converted to electricity by a generator driven by the mechanical energy. Such systems are commonplace in marine, freight, and industrial applications and are commonly used in applications where the electrical load is relatively constant, and the load response of the generator set has little effect on the proper operation of the vehicle.
Turbine-electric powertrains represent a similar method of operation as petrol-electric and diesel-electric powertrains, but implement a gas turbine as the prime mover.
In other applications, such as electric UAV applications, batteries are most commonly employed due to their ease of use and, with certain battery chemistries with high discharge capabilities, their ability to supply large amounts of power and rapidly meet a variable power demand. Systems such as UAVs can have rapidly changing electrical load requirements which the powertrain must be able to accommodate for proper operation and this is especially true for UAVs, where the vehicle is inherently unstable and relies on the rapid response of the powertrain to stay airborne and stable and batteries have been the preferred power system solution.
Despite these advantages, several problems exist with the use of batteries as the energy source in a powertrain system. For example, batteries are typically manufactured with specific chemistries that are a compromise between energy density, capacity, expected lifetime (rechargability and robustness), weight, safety (flammability, chemical reactivity), expense, expected operating temperature range, etc.
In particular, even the best currently available batteries offer a very low gravimetric energy density relative to most combustion fuels. Using UAV platforms as an example, the low gravimetric energy density of even the best (and often most expensive) batteries unduly limit payload capacities and flight times, making UAVs unusable in applications for which they would otherwise be well suited. Similar problems exist with the use of batteries in the automotive field, for example the range of Tesla™ battery powered vehicles is generally much less than comparable vehicles powered by combustion engines.
Prior attempts to address the limitations of battery powered powertrains have included hybrid powertrain systems which employ a combination of batteries and combustion fuel energy sources. In hybrid powertrain systems, a battery is combined with an electric generator and combustion engine. There are a variety of operating strategies for such hybrid powertrain systems, but to some extent, all of these strategies involve operating the combustion engine to produce an electric power output that can then be applied to electric drive motors in combination with the output of the battery, or to directly recharge the battery.
Because these powertrains use the generator as a complement to the battery system, the batteries in these systems are recharged and discharged frequently and are often responsible for meeting all (or most) of the vehicle's power demands for a significant period of time. This necessitates a battery with a very large capacity and with a battery chemistry that makes them unsuitable for applications where weight and cost is a significant concern.